Very stupid and insane possibly — one must be kept in (7)
I believe the answer is:
asinine
'very stupid' is the definition.
(very foolish)
'insane possibly one must be kept in' is the wordplay.
'possibly' indicates anagramming the letters.
'one' becomes 'I' (Roman numeral).
'must be kept in' indicates putting letters inside.
'insane' anagrammed gives 'asinne'.
'asinne' going around 'i' is 'ASININE'.
'and' is the link.
(Other definitions for asinine that I've seen before include "simple" , "Daft" , "Foolish like donkey" , "Is Annie? (Anag)" , "Foolish, silly" .)